The invention relates generally to display devices, and, more particularly, to a method and system in which the electro-optical material of the spatial light modulator used in a display device is illuminated with pulses of light delayed by a time equal to the measured response time of the electro-optical material to linearize the grey scale of the display device.
Recently, display devices based on electro-optical materials such as ferroelectric liquid crystal materials have been introduced. Such display devices can form part of a miniature, wearable display, sometimes called an eyeglass display, and also can form part of a front- or rear-projection display. FIG. 1 shows an example of a display device 1. The display device is composed of reflective light valve 10, light source 15, which generates light that illuminates the light valve, and output optics 23 that focus the light to form an image (not shown). The light valve is composed of reflective spatial light modulator 25, polarizer 17, beam splitter 19 and analyzer 21. The display device shown may form part of a miniature, wearable display, a projection display or other types of displays.
In display device 1, light generated by light source 15 passes through polarizer 17. The polarizer polarizes the light output from the light source. Beam splitter 19 reflects a fraction of the polarized light output from the polarizer towards spatial light modulator 25. The spatial light modulator is divided into a two-dimensional array of picture elements (pixels) that define the pixels of the display device. In this disclosure, the term display pixel X will be used as a abbreviation for the term pixel of the display device defined by pixel X of spatial light modulator. The beam splitter transmits a fraction of the light reflected by the spatial light modulator to analyzer 21.
The direction of an electric field in each pixel of spatial light modulator 25 determines whether or not the direction of polarization of the light reflected by the pixel is rotated by 90xc2x0 relative to the direction of polarization of the incident light. The light reflected by each pixel of the spatial light modulator passes through beam splitter 19 and analyzer 21 and is output from the light valve depending on whether or not its direction of polarization was rotated by the spatial light modulator. The light output from light valve 10 passes to output optics 23.
Light source 15 may be composed of LEDs. The LEDs are of three different colors in a color display. Other light-emitting devices whose output can be rapidly modulated may alternatively be used as light source 15. As a further alternative, a white light source and a light modulator (not shown) may be used. The light modulator modulates the amplitude of the light generated by the light source to define the illumination period and balance period of the spatial light modulator. In a light valve for use in a color display device, the light modulator additionally modulates the color of the light output from the light source.
Polarizer 17 polarizes the light generated by light source 15. The polarization is preferably linear polarization. Beam splitter 19 reflects the polarized light output from the polarizer towards spatial light modulator 25, and transmits the polarized light reflected by the spatial light modulator to analyzer 21. The direction of maximum transmission of the analyzer is orthogonal to that of the polarizer in this example.
Spatial light modulator 25 is composed of transparent electrode 33 deposited on the surface of transparent cover 37, reflective electrode 35 located on the surface of semiconductor substrate 39, and layer 31 of electro-optical material sandwiched between the transparent electrode and the reflective electrode. The reflective electrode is divided into a two-dimensional array of pixel electrodes that define the pixels of the spatial light modulator and of the light valve. A substantially-reduced number of pixel electrodes is shown to simplify the drawing. For example, in a light valve for use in a large-screen computer monitor, the reflective electrode could be divided into a two-dimensional array of 1600xc3x971200 pixel electrodes. An exemplary pixel electrode is shown at 41. Each pixel electrode reflects the portion of the incident polarized light that falls on it towards beam splitter 19.
A pixel drive circuit applies a pixel drive signal to the pixel electrode of each pixel of spatial light modulator 25. Pixel drive circuit 44 of exemplary pixel 42 is shown in this example as being located in semiconductor substrate 39. The pixel drive signal alternates between two different voltage levels, a high state and a low state. When a liquid crystal material is used as the electro-optical material of layer 31, transparent electrode 33 is maintained at a fixed potential mid-way between the voltage levels of the pixel drive signal. The potential difference between the pixel electrode and the transparent electrode establishes an electric field across the part of liquid crystal layer 31 between the pixel and transparent electrodes. The direction of the electric field determines whether the liquid crystal layer rotates the direction of polarization of the light reflected by the pixel electrode, or leaves the direction of polarization unchanged.
When display device 1 forms part of a miniature, wearable display, output optics 23 are composed of an eyepiece that receives the light reflected by reflective electrode 35 and forms a virtual image at a predetermined distance in front of the viewer (not shown). In a cathode-ray tube replacement or in a projection display, the output optics are composed of projection optics that focus an image of the reflective electrode on a transmissive or reflective screen (not shown). Optical arrangements suitable for use as an eyepiece or projection optics are well known in the art and will not be described here.
Since the direction of maximum transmission of analyzer 21 is orthogonal to the direction of polarization defined by polarizer 17, light whose direction of polarization has been rotated through 90xc2x0 by a pixel of spatial light modulator 25 will pass through the analyzer and be output from light valve 10 whereas light whose direction of polarization has not been rotated will not pass through the analyzer. The analyzer only transmits to output optics 23 light whose direction of polarization has been rotated by pixels of the spatial light modulator. The direction of the electric field applied to each pixel of the spatial light modulator determines whether the corresponding display pixel will appear bright or dark. When a display pixel appears bright, it will be said to be ON, and when the display pixel appears dark, it will be said to be OFF.
The direction of maximum transmission of analyzer 21 can alternatively be arranged parallel to that of polarizer 17, and a non-polarizing beam splitter can be used as beam splitter 19. In this case, spatial light modulator 25 operates in the opposite sense to that just described.
To produce the grey scale required by to display an image notwithstanding the binary optical characteristics of the display pixels, the apparent brightness of each display pixel is varied by temporally modulating the direction of polarization of the light reflected by the corresponding pixel of spatial light modulator 25. This, in turn, temporally modulates the light output by the corresponding display pixel. The light is modulated by defining a basic time period that will be called the illumination period of the spatial light modulator. The pixel electrode is driven by the pixel drive signal that switches the pixel from ON to OFF. The fraction of the illumination period in which the pixel is in its ON state determines the apparent brightness of the display pixel.
Ferroelectric liquid crystal-based spatial light modulators suffer the disadvantage that, after each time the pixel drive signal has been applied to the pixel electrode to cause the pixel to change the direction of polarization of the light reflected by it, the DC balance of the pixel must be restored. This is done by defining a second basic time period called the balance period, equal in duration to the illumination period, and driving the pixel electrode with a complementary pixel drive signal having high state and low state durations that are complementary to the high state and low state durations of the pixel drive signal during the illumination period. The illumination period and the balance period collectively constitute a display period.
To prevent the complementary pixel drive signal from causing the display device to display a substantially uniform, grey image, light source 15 illuminating light valve 10 is modulated so that the light valve is illuminated with a pulse of light having a duration equal to that of the illumination period, and is not illuminated during the balance period. Modulating the light source as just described to illuminate the light valve with pulses of light reduces the light throughput of the light valve to about half of that which could be achieved if DC balance restoration were unnecessary.
The pixel drive circuit of each pixel of spatial light modulator 25 determines the duration of the ON state of the corresponding display pixel in response to a portion of input video signal 43 corresponding to the location of the pixel in the spatial light modulator.
FIGS. 2A-2E illustrate the operation of exemplary pixel 42 of spatial light modulator 25 shown in FIG. 1 during three consecutive display periods. Pixel 42 is controlled by pixel electrode 41. The remaining pixels operate similarly. FIG. 2A shows each display period composed of an illumination period (ILLUM) and a balance period (BALANCE) having equal durations. Each display period may correspond to one frame of input video signal 43, for example, as shown. As another example, each display period may correspond to a fraction of one frame of the input video signal.
FIG. 2B shows the pixel drive signal applied to exemplary pixel electrode 41. Transparent electrode 33 is held at a voltage level of V/2, so that changing the voltage level on the pixel electrode from 0 to V reverses the direction of the electric field applied to layer 31 of ferroelectric liquid crystal material. The level of the pixel drive signal is V for a first temporal portion 1 TP of each illumination period. The level of the pixel drive signal is 0 for the second temporal portion 2 TP constituting the remainder of the illumination period, and also for the first temporal portion 1TP of the subsequent balance period. The first temporal portion of the balance period has a duration equal to the first temporal portion of the illumination period. However, the level of the pixel drive signal is 0 during the first temporal portion of the balance period, whereas the level of the drive signal is V during the first temporal portion of the illumination period. Finally, the level of the pixel drive signal changes to V for the second temporal portion 2 TP constituting the remainder of the balance period. Consequently, during the balance period, the level of the pixel drive signal is 0 and V for times equal to the times that it was at V and 0, respectively, during the illumination period. As a result, when the transparent electrode 33 is held at a constant voltage equal to V/2, the electric field applied to the liquid crystal material of the pixel averages to zero over the display period.
In the example shown, the duration of the first temporal portion 1 TP of the drive signal is different in each of the three illumination periods. The duration of the first temporal portion, and, hence, of the second temporal portion, of each illumination period depends on the voltage level of the corresponding sample of input video signal 43.
FIG. 2C shows the effect of spatial light modulator 25 on the direction of polarization of the light impinging on analyzer 21. The direction of polarization is indicated by the absolute value of the angle xcex1 between direction of polarization of the light impinging on the analyzer and the direction of maximum transmissivity of the analyzer. The analyzer transmits light having an angle xcex1 close to zero and absorbs light having an angle xcex1 close to 90xc2x0. In each display period, the angle xcex1 has values corresponding to the display pixel being bright and dark for equal times due to the need to restore the DC balance of the pixel.
FIG. 2D shows the modulation of light source 15. The light source is ON throughout the illumination period of each display period, and is OFF during the following balance period.
FIG. 2E shows the light output from display pixel 42, i.e., the pixel of the display device corresponding to exemplary pixel 42 of spatial light modulator 25. Light is output from the display pixel only during the first temporal portion of the illumination period of each display period. No light is output during the second temporal portion of the illumination period. Moreover, no light is output during the balance period of the display period because the light source 15 is OFF during the balance period.
In the spatial light modulator just described, a bistable electro-optical material can be used instead of the conventional ferroelectric liquid crystal material as layer 31 of electro-optical material. Whether or not the bistable electro-optical material rotates the direction of polarization of the light reflected by the pixel electrode is set by applying a short-duration electrical pulse of the required polarity to the pixel electrode. The bistable electro-optical material will retain the optical property set by the electrical pulse until the material is reset by applying a short duration optical pulse of the opposite polarity. Bistable electro-optical materials have the advantage that they can be driven by a pixel drive signal that is inherently DC balanced, so the balance period described above need not be provided. Consequently, using a bistable electro-optical material provides a larger luminous efficiency than using an electro-optical material that requires a non-illuminated balance period because the spatial light modulator can be illuminated for most of the display period.
Moreover, much of the disadvantage in luminous efficiency of electro-optical materials that require a DC balance period can be overcome by adding a switchable polarization inverter to the optical path between spatial light modulator 25 and beam splitter 19. This allows the spatial light modulator to be illuminated with a pulse of light in each illumination period and a pulse of light in each balance period. Blanking periods between the light pulses provide time for the polarization inverter to switch. A display device using this technique is described in U.S. patent application Ser. No. 09/183,554, assigned to the assignee of this disclosure and incorporated in this disclosure by reference. The polarization inverter is switched to one of its states during the illumination period and to the other of its states during the balance period. The polarization inverter inverts the negative image that would otherwise be generated during the balance period.
FIGS. 2F and 2G illustrate the operation of a display device that incorporates a polarization inverter. The pixel drive signal is as shown in FIG. 2B. FIG. 2F shows the light output by light source 15. The light source illuminates spatial light modulator 10 with a pulse of light during each of the illumination period and the balance period. The display device that includes the polarization inverter therefore outputs light during both the illumination period and the balance period, as shown in FIG. 2G.
In the example of the micro-display described above with reference to FIGS. 2A-2E, the pixel drive signal switches the pixel ON prior to the beginning of the illumination period, as shown in FIG. 2B in which the display pixel is in its ON state when the pixel electrode is at voltage V, and light source 15 illuminates spatial light modulator 25 during each illumination period. Then, at a later time during the illumination period, the pixel drive signal reverts to its original state to switch the display pixel OFF. The fraction of the illumination period during which the display pixel was ON sets the apparent brightness of the pixel. Despite being switched ON before the beginning of the illumination period, the display pixel is not illuminated prior to the beginning of the illumination period and, hence, does not appear bright until the beginning of the illumination period.
FIGS. 2A and 2B respectively show the change in state of the pixel drive signal and the resulting change in the direction of polarization of the light reflected by the pixel of spatial light modulator 25 occurring synchronously. In a practical example, the optical properties of electro-optical materials in general and of liquid crystal materials in particular do not switch instantaneously as shown in FIG. 2B in response to a change in state of the pixel drive signal. Instead, there is a delay between the time when the pixel drive signal changes state and when the optical property of the electro-optical material changes state in response to the pixel drive signal. This delay will be called the response time of the electro-optical material. Since the delay may differ depending on the direction in which the optical property of the electro-optical material changes, the electro-optical material is characterized by two response times, one for each direction. In this disclosure, the response time relating to the change in the optical property that causes the display pixel to change from bright to dark will be called the bright-to-dark response time, and that relating to the change in the optical property that causes the display pixel to change from dark to bright will be called the dark-to-bright response time.
FIG. 3A shows change 51 in the state of the pixel drive signal and FIG. 3B shows the delay in resulting change 52 in the optical property of the electro-optical material, namely, the direction of polarization of the light reflected by the pixel of the spatial light modulator. This delay is determined by the bright-to-dark response time of the electro-optical material. FIG. 3B also shows the delay in change 56 in the optical property of the electro-optical material resulting from change 54 in the state of the pixel drive signal shown in FIG. 3A. This delay is determined by the bright-to-dark-to-bright response time of the electro-optical material.
The display device 1 operates on the basic assumption that the duty cycle of the pixel drive signal is proportional to the pixel value fed to the pixel and that the apparent brightness of the corresponding display pixel is proportional to the duty cycle of the pixel drive signal. Thus, the apparent brightness of the display pixel is proportional to the pixel value. However, the delay in the optical properties of spatial light modulator 25 changing state caused by the response times of the electro-optical material results in a non-linear relationship between the duty cycle of the pixel drive signal and the apparent brightness of the corresponding display pixel. In the example just described in which the display pixel is turned OFF during the illumination period, the delay Db in the optical property of the spatial light modulator changing state makes the display pixel appear brighter than the brightness defined by the pixel value that defines the duty cycle of the pixel drive signal. This effect is especially severe at apparent brightnesses near black level. The effect of the response times of the electro-optical material on the linearity of the grey scale displayed by the display pixels can be corrected by delaying the illumination of the spatial light modulator by a time equal to the respective response time of the electro-optical material.
One way of compensating for the response time of the electro-optical material of the spatial light modulator is to program the response time into a configuration register that is part of the firmware that controls light source 15 of the display device. However, the response time of the electro-optical material can vary from batch-to-batch and can vary dynamically with such factors as temperature and the age of the electro-optical material. It is difficult to compensate for the above-mentioned dynamic variations in the response time of the electro-optical material using firmware programming. Furthermore, the need for additional firmware to program the response time increases the complexity of the display device.
What is needed, therefore, is a dynamic, accurate, on-chip method and system for dynamically determining response time of the electro-optical material to allow the illumination of the spatial light modulator to be delayed by the appropriate amount.
The invention provides a method of illuminating a layer of electro-optical material with pulses of light. In the method, a monitor pixel that includes a portion of the layer of electro-optical material is provided. A measured response time of the electro-optical material is measured using the monitor pixel. Illumination of the layer of the electro-optical material is then delayed by a time corresponding to the measured response time of the electro-optical material.
The invention also provides a system for illuminating a layer of electro-optical material with pulses of light. The system comprises a monitor pixel, a monitor pixel driver, an optical response detector and an illumination control circuit. The monitor pixel includes a portion of the layer of electro-optical material. The monitor pixel driver is configured to drive the monitor pixel with a monitor pixel drive signal. The optical response detector is coupled to the monitor pixel and is configured to generate a detection signal indicating a change in an optical property of the monitor pixel. The response time measurement circuit is configured to measure a measured response time of the electro-optical material in response to the monitor pixel drive signal and the detection signal. The illumination control circuit is configured to delay illumination of the layer of the electro-optical material by a time corresponding to the measured response time of the electro-optical material.
In both the method and system according to the invention, current flowing into or out of the monitor pixel may be used to measure the measured response time of the electro-optical material.
An advantage of the invention is that it enables the pulses of light illuminating the electro-optical material to be delayed in accordance with the actual, measured response time of the electro-optical material. Thus, a display device incorporating the electro-optical material can display an accurate and linear grey scale notwithstanding batch-to-batch variations in the response time of the electro-optical material, and changes in the response time caused by such factors as temperature and aging.
Another advantage of the invention is that it improves the efficiency of a display element employing the invention since it allows the electro-optical material to be illuminated for a greater fraction of each illumination period and, optionally, balance period.
Another advantage of the invention is that it can be embodied in circuits fabricated in the same semiconductor substrate as the pixel drive circuits and auxiliary circuits of the spatial light modulator of which the layer of electro-optical material is part.
Another advantage of the invention is that it improves the image quality provided by a display element in which it is incorporated.
Another advantage of the invention is that it is simple in design and easily implemented on a mass scale for commercial production.
Other features and advantages of the invention will become apparent upon examination of the following drawings and detailed description.